Many services on the Internet, such as email, online polls, weblogs, and message boards, are provided free of charge to users. However, the freeness of these services leaves them vulnerable to abuse. The abuse often involves malicious users deploying automated scripts or software agents known as bots to use the services in order to send spam, engage in ballot stuffing, and so forth.
Human interaction proofs have been deployed to combat such abuse. Human interaction proofs are used to verify that the user is a human being before granting the user access. A CAPTCHA (“completely automated public Turing test to tell computers and humans apart”) is an example of a human interaction proof. A commonly deployed CAPTCHA is one that asks the user to identify or recognize text characters in an image, where the text characters are visually obscured or distorted. However, much progress has been made in the development of algorithms and programs that can solve such text identification or character recognition CAPTCHAs. As a result, the deterrence effect of such CAPTCHAs has been weakened.
Accordingly, there is a need for a human interaction proof that is less vulnerable to attack.